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<H1>IBM Visualization Data Explorer Programmer&#39;s Reference</H1>
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<HR>
<H1><A NAME="HDRPICKST" HREF="../proguide.htm#ToC_68">Chapter 9. Using the Pick
Structure</A></H1>
<P><A NAME="PToC11" HREF="../proguide.htm#ToC">Partial Table-of-Contents</A>
<MENU>
<LI><A NAME="PToC_69" HREF="#HDRPKSE">9.1 The Pick Structure</A>
<LI><A NAME="PToC_70" HREF="progu034.htm#HDRSHOWP">9.2 ShowPick Module
Example--Using Color to Show a Picked Object</A>
</MENU><HR><P>
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<P>
Data Explorer includes a tool for "picking" points in Objects
in an image.
<P>
This tool, Pick, creates a structure that can be used to perform various
functions (e.g., to display the data value at picked points).
But you can also write your own module to perform different functions if
you like.
<HR>
<H2><A NAME="HDRPKSE" HREF="#PToC_69">9.1 The Pick Structure</A></H2>
<A NAME="IDX258"></A>
<P>
Picking is the selection of a location on an object in an image by use
of the mouse.
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A straight line from the camera through the location selected constitutes
a "poke," which may intersect the object in the image in one or
more places or in none at all.
The intersections are called "picks."
For example, a poke through a spherical isosurface results in two
picks:
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one on the "front" surface and one on the "back".
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Picks differ from probes in that probes may be present anywhere in
3-dimensional space, picks only on the surface of
an object.
<P>
The pick structure is a Field, and the picked points are listed in its
"positions" component.
A number of routines in Data Explorer allow you to query the pick structure
output by the Pick tool and to traverse a picked Object.
(See <A HREF="progu071.htm#HDRPIES">14.6 , "Pick-Assistance Routines"</A> for
details.)
The structure includes information on how to traverse the picked Object
to reach the picked element.
It also identifies:
<UL COMPACT>
<LI>the connection in which the picked point resides (the element ID)
<LI>the vertex of the picked element closest to the picked point
(the vertex ID)
<LI>the position of the picked point itself.
</UL>
<P>
If the picked Object has no connections, the element ID and the vertex
ID both refer to the position closest to the
picked point.
Other information can be accessed with pick-assistance routines.
<P><B>Note: </B>For a module that uses the pick structure, the Object displayed
in the image being picked must (1) be the same as the Object
passed to the module or (2) have a matching
Object hierarchy.
The reason for this requirement is that the output of the Pick tool
describes the location of the picked Object as it exists in the
hierarchy of the rendered Object.
To use the pick structure, therefore, requires an Object with a
matching structure.
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